Quantitation and analysis of droplets ejected from an inkjet device

ABSTRACT

A method for determining either the mass of one or more drops dispensed from an inkjet dispensing device or the concentration of dissolved solutes form one or more drops of a liquid of interest dispensed from an inkjet dispensing device utilizes UV visible spectroscopy. The UV absorption spectra of the constituents of the solution may be compared to predetermined calibration curves to accurately determine mass and concentrations of a single drop.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of determining the mass ofdrops or droplets ejected from an inkjet device or dispenser, and moreparticularly, to a method for determining the mass of drops or dropletsas well as assessing the mixing effects of solutions ejected from aninkjet dispenser utilizing UV visible spectroscopy.

2. Discussion of the Related Art

In an increasingly large number of industries, the ability to accuratelyand repeatedly deposit nanogram quantities of a given substance iscritical to the development of new technologies. This is largely drivenby a move towards micro- and nano-scale products that require extremelyaccurate processing steps. Many applications require repeatabledeposition of nano- or picoliter quantities of solutions to preciselocations on a target. This is particularly true in the manufacturing ofmany medical devices where the amount and location of drug loading mustbe controlled to very precise specifications. In such cases,drop-on-demand inkjet technology is an attractive choice as it addressesthe needs for both accurate targeting and repeatable droplet ejection.

Particularly for these kinds of highly-controlled applications, thequantity of substance being ejected from the inkjet devices must beknown to extreme accuracy. Various methods have been described todetermine the quantity of substance, including the use of atomic forcemicroscopy cantilevers, quartz crystal microbalances, nanomechanicalresonators and gravimetry. However, all of these methods require eitherhighly sensitive, time-consuming calibration processes that areimpractical for a manufacturing process application or a large number ofdrops to ensure an accurate measurement. A system that can quantify thematerial dispensed in small drop numbers, requires little calibrationand can be easily integrated into an existing process is of importancein many industries. UV-visible spectroscopy meets these criteria due toits sensitive detections limits, relatively simple calibration and shortsampling time.

Despite UV-visible spectroscopy's relative weakness in identifyingunknown compounds, its ability to quantify known substances in solutionis quite robust. As such, it is a common choice for applications inwhich a known substance is dissolved in a known solvent and onlydetermination of the concentration is desired. Further, in mixedsolutions with more than one component, absorbance values measured atmultiple wavelengths may be compared to determine relativeconcentrations of individual components, which is useful for assessmentsof solution mixing and degradation of individual components.

Accordingly, there exists a need for overcoming the disadvantagesassociated with the current technology by developing a method ofdetermining the mass of individual and small quantities of dropletsejected from an inkjet device and a method for quantitative assessmentsof mixing effects of solutions dispensed from an inkjet device.

SUMMARY OF THE INVENTION

The method of quantitation and analysis of droplets ejected from aninkjet device in accordance with the present invention overcomes thelimitations with the prior art as briefly described above.

In accordance with one aspect, the present invention is directed to amethod for determining at least one of the drop mass or concentration ofdissolved solutes for one or more drops of a liquid of interestdispensed from an inkjet dispensing device. The method comprisingdepositing one or more drops of a first solvent from an inkjetdispensing device into a cuvette containing a predetermined amount of asecond solvent and mix, determining the UV-absorption spectra of theresulting solution of the first and second solvents in the cuvette, andcalculating the mass of the one or more drops dispensed by the inkjetdispensing device by utilizing the UV-absorption spectra at the specificwavelength that corresponds to the first solvent and comparing it to apredetermined calibration curve.

In accordance with another aspect, the present invention is directed toa method for determining at least one of the drop mass or concentrationof dissolved solutes for one or more drops of a liquid of interestdispensed from an inkjet dispensing device, the method comprisingdepositing one or more drops of a liquid of interest comprising a firstsolvent and one or more solutes from an inkjet dispensing device into acuvette containing a predetermined amount of a second solvent and mixinto a resulting solution, determining the UV-absorption spectra of theresulting solution in the cuvette, calculating the mass of the one ormore drops dispensed by the inkjet dispensing device by utilizing theUV-absorption spectra at the specific wavelength that corresponds to thefirst solvent and comparing it to a predetermined calibration curve, andcalculating the concentration of the one or more solutes in each of theone or more drops dispensed by the inkjet dispensing device by utilizingthe UV-absorption spectra at the specific wavelengths that correspond tothe one or more solutes in the solution and comparing it topredetermined calibration curves.

The exemplary method of determining the mass of drops ejected from aninkjet dispenser in accordance with the present invention includesdispensing a known, finite number of drops into a known volume ofsolvent and measuring the absorbance of this final solution usingUV-visible spectroscopy. The solution dispensed from the inkjetdispenser may be either a pure substance or a solution, in which caseany of the materials in solution may be measured by the exemplary methodof the present invention. The exemplary method of the present inventionutilizes the ability of UV-visible spectroscopy to accurately andprecisely measure the absorbance of a solution, which may be correlatedto sample concentration. After the drops are ejected into the solvent ofchoice, the absorbance of the solution is measured at wavelengthsassociated with the components of the solution being dispensed from theinkjet. This allows for an immediate measurement of the volume dispensedfrom the inkjet broken down by individual components. Because eachcomponent may be measured simultaneously and independent of the others,this methodology also allows for quantitative assessments of solutionmixing effects.

The exemplary method of the present invention is not only beneficial forresearch and development purposes, but is also suitable formanufacturing environments, allowing for immediate, accuratemeasurements of inkjet behavior. Further, the exemplary method's use ofnon-volatile solvents for sample collection ameliorates problemsassociated with measurement of volatile solutions, as are often used ininkjet applications.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the invention will beapparent from the following, more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

FIG. 1 is a perspective view of a therapeutic agent delivery device inthe form of an expandable stent.

FIG. 2 is a cross-sectional view of a portion of a therapeutic agentdelivery device having a beneficial agent contained in an opening inlayers.

FIG. 3 is a side view of a piezoelectric micro-jetting dispenser fordelivery of a beneficial agent.

FIG. 4 is a cross-sectional view of an expandable medical device on amandrel and a piezoelectric micro-jetting dispenser.

FIG. 5 is a perspective view of a system for loading an expandablemedical device with a beneficial agent.

FIG. 6 is a perspective view of a bearing for use with the system ofFIG. 5.

FIG. 7 is a side cross-sectional view of an acoustic dispenser fordelivery of a beneficial agent to an expandable medical device.

FIG. 8 is a side cross-sectional view of an alternative acousticdispenser reservoir.

FIG. 9 is a side cross-sectional view of an alternative piezoelectricdispenser system.

FIG. 10 is a diagrammatic representation of an exemplary waveform forcontrolling an inkjet dispenser with input parameters labeled inaccordance with the present invention.

FIG. 11 is a diagrammatic representation of the electronics required todispense a desired number of sequences of drops in accordance with thepresent invention.

FIG. 12 are high speed images captured with a shutter speed of 2microseconds at a rate of 2,800 fps showing the dissimilarity betweendrops in a sequence in accordance with the present invention.

FIG. 13 is a plot of the results of image analysis for high speedvideography of 25 sets of bursts of 5 drops with adjacent burstsseparated by 30 microseconds in accordance with the present invention.

FIG. 14 is a plot of the average drop weight as a function of drivingamplitude for the different numbers of drops in a sequence in accordancewith the present invention.

FIG. 15 is a plot of the average drop mass as a function of quantity ofdrops in a burst in Region A as defined in FIG. 14 in accordance withthe present invention.

FIG. 16 is a plot of the average mass per drop for sequences of varyingdrop numbers in Region C as defined in FIG. 14 in accordance with thepresent invention.

FIG. 17 is a plot of the drop mass as a function of order of ejectionwithin a burst in accordance with the present invention.

FIG. 18 is a plot of the average drop mass for an entire sequence ofdrops as a function of time between adjacent bursts in accordance withthe present invention.

FIG. 19 is a simplified schematic of a single channel inkjet device inaccordance with the present invention.

FIG. 20 is a diagrammatic representation of a plurality of waveformswith pulses of different amplitudes in accordance with the presentinvention.

FIG. 21 is a plot of the UV-visible spectra of various concentrations ofDMSO dissolved in de-ionized water in accordance with the presentinvention.

FIG. 22 is a plot of UV-visible absorbance spectrum of a ten (10)microgram per milliliter solution of DMSO, sirolimus and PLGA inde-ionized water in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a method for determining the mass ofindividual and small quantities of droplets and for quantitativeassessments of mixing effects of solutions dispensed from an inkjetdevice. The method of the present invention allows for the accurate andrepeatable deposition of small quantities of material at a targetlocation such as for loading a beneficial agent into an expandablemedical device.

The term “beneficial agent” as used herein is intended to have itsbroadest possible interpretation and is used to include any therapeuticagent or drug, as well as inactive agents such as barrier layers,carrier layers, therapeutic layers, protective layers or combinationsthereof.

The terms “drug” and “therapeutic agent” are used interchangeably torefer to any therapeutically active substance that is delivered to abodily conduit of a living being to produce a desired, usuallybeneficial, effect. The present invention is particularly well suitedfor the delivery of antineoplastic, angiogenic factors,immuno-suppressants, anti-inflammatories and antiproliferatives(anti-restenosis agents) such as paclitaxel and Rapamycin for example,and antithrombins such as heparin, for example.

The term “matrix” or “biocompatible matrix” are used interchangeably torefer to a medium or material that, upon implantation in a subject, doesnot elicit a detrimental response sufficient to result in the rejectionof the matrix. The matrix typically does not provide any therapeuticresponses itself, though the matrix may contain or surround atherapeutic agent, a therapeutic agent, an activating agent or adeactivating agent, as defined herein. A matrix is also a medium thatmay simply provide support, structural integrity or structural barriers.The matrix may be polymeric, non-polymeric, hydrophobic, hydrophilic,lipophilic, amphiphilic, and the like.

The term “bioresorbable” refers to a matrix, as defined herein that canbe broken down by either chemical or physical process, upon interactionwith a physiological environment. The bioresorbable matrix is brokeninto components that are metabolizable or excretable, over a period oftime from minutes to years, preferably less than one year, whilemaintaining any requisite structural integrity in that same time period.

The term “polymer” refers to molecules formed from the chemical union oftwo or more repeating units, called monomers. Accordingly, includedwithin the term “polymer” may be, for example, dimers, trimers andoligomers. The polymer may be synthetic, naturally-occurring orsemisynthetic. In preferred form, the term “polymer” refers to moleculeswhich typically have a M_(w) greater than about 3000 and preferablygreater than about 10,000 and a M_(w) that is less than about 10million, preferably less than about a million and more preferably lessthan about 200,000.

The term “openings” refers to holes of any shape and includes boththrough-openings, blind holes, slots, channels and recesses.

The term “shot” or “drop” herein refers to the material ejected from aninkjet dispenser, inkjet, or micro-jetting dispenser as a result of asingle voltage pulse to the piezoelectric element within the inkjet.After the material is ejected from the inkjet, it may fragment intosmaller masses herein referred to as “droplets”. In addition, the termsinkjet dispenser, inkjet, inkjet dispensing unit, micro-jettingdispenser and the like may be used interchangeably.

FIG. 1 illustrates a medical device 10 according to the presentinvention in the form of a stent design with large, non-deforming struts12 and links 14, which may contain openings (or holes) 20 withoutcompromising the mechanical properties of the struts or links, or thedevice as a whole. The non-deforming struts 12 and links 14 may beachieved by the use of ductile hinges which are described in detail inU.S. Pat. No. 6,241,762 which is incorporated hereby by reference in itsentirety. The holes 20 serve as large, protected reservoirs fordelivering various beneficial agents to the tissue in the area of thetissue in the area of the device implantation site.

As shown in FIG. 1, the openings 20 may be circular 22, rectangular 24,or D-shaped 26 in nature and form cylindrical, rectangular, or D-shapedholes extending through the width of the medical device 10. It may beappreciated that the openings 20 may be other shapes without departingfrom the present invention. In addition, the holes or reservoirs do nothave to be through holes as described above.

The volume of beneficial agent that may be delivered using openings 20is about 3 to 10 times greater than the volume of a 5 micron coatingcovering a stent with the same stent/vessel wall coverage ratio. Thismuch larger beneficial agent capacity provides several advantages. Thelarger capacity may be used to deliver multi-drug combinations, eachwith independent release profiles, for improved efficacy. Also, largercapacity can be used to provide larger quantities of less aggressivedrugs and to achieve clinical efficacy without the undesirableside-effects of more potent drugs, such as retarded healing of theendothelial layer.

FIG. 2 shows a cross-section of a medical device 10 in which one or morebeneficial agents have been loaded into the opening 20 in layers.Examples of some methods of creating such layers and arrangements oflayers are described in U.S. Pat. No. 7,208,010, issued on Apr. 24,2007, which is incorporated herein by reference in its entirety.Although the layers are illustrated as discrete layers, the layers canalso mix together upon delivery to result in an inlay of beneficialagent with concentration gradients of therapeutic agents but withoutdistinct boundaries between layers.

According to one example, the total depth of the opening 20 is about 100to about 140 microns, typically 125 microns and the typical layerthickness would be about 2 to about 50 microns, preferably about 12microns. Each typical layer is thus individually about twice as thick asthe typical coating applied to surface-coated stents. There would be atleast two and preferably about ten to twelve such layers in a typicalopening, although this amount may be tailored to the particular need,with a total beneficial agent thickness about 25 to 28 times greaterthan a typical surface coating. According to one preferred embodiment ofthe present invention, each of the openings has an area of at least5×10⁻⁶ square inches, and preferably at least 7×10⁻⁶ square inches.Typically, the openings are filled about 50 percent to about 75 percentfull of beneficial agent.

Since each layer is created independently, individual chemicalcompositions and pharmacokinetic properties can be imparted to eachlayer. Numerous useful arrangements of such layers can be formed, someof which will be described below. Each of the layers may include one ormore agents in the same or different proportions from layer to layer.The layers may be solid, porous, or filled with other drugs orexcipients. As mentioned above, although the layers are depositedseparately, they may mix forming an inlay without boundaries betweenlayers, potentially resulting in a transition gradient within the inlay.

As shown in FIG. 2, the opening 20 is filled with a beneficial agent.The beneficial agent includes a barrier layer 40, a therapeutic layer30, and a cap layer 50.

Alternatively, different layers could be comprised of differenttherapeutic agents altogether, creating the ability to release differenttherapeutic agents at different points in time. The layers of beneficialagent provide the ability to tailor a delivery profile to differentapplications. This allows the medical device according to the presentinvention to be used for delivery of different beneficial agents to awide variety of locations in the body.

A protective layer in the form of a cap layer 50 is provided at a tissuecontacting surface of a medical device. The cap layer 50 can block orretard biodegradation of subsequent layers and/or blocks or retardsdiffusion of the beneficial agent in that direction for a period of timewhich allows the delivery of the medical device to a desired location inthe body. When the medical device 10 is a stent which is implanted in alumen, the barrier layer 40 is positioned on a side of the opening 20facing the inside of the lumen. The barrier layer 40 prevents thetherapeutic agent 30 from passing into the lumen and being carried awaywithout being delivered to the lumen tissue. Alternately, there may beinstances where preferential directional drug delivery into the lumen iswarranted, in those cases the barrier layer 40 may be positioned on aside of the openings 20 facing the tissue, thus preventing thetherapeutic agent 30 from facing into the tissue.

Typical formulations for therapeutic agents incorporated in thesemedical devices are well known to those skilled in the art.

Although the present invention has been described with reference to amedical device in the form of a stent, the medical devices of thepresent invention can also be medical devices of other shapes useful forsite-specific and time-release delivery of drugs to the body and otherorgans and tissues. The drugs may be delivered to the vasculatureincluding the coronary and peripheral vessels for a variety oftherapies, and to other lumens in the body. The drugs may increase lumendiameter, create occlusions, or deliver the drug for other reasons.

Medical devices and stents, as described herein, are useful for theprevention of amelioration of restenosis, particularly afterpercutaneous transluminal coronary angioplasty and intraluminal stentplacement. In addition to the timed or sustained release ofanti-restenosis agents, other agents such as anti-inflammatory agentsmay be incorporated into the multi-layers incorporated in the pluralityof holes within the device. This allows for site-specific treatment orprevention any complications routinely associated with stent placementsthat are known to occur at very specific times after the placementoccurs.

FIG. 3 shows a piezoelectric micro-jetting dispenser 100 used todispense a beneficial agent into the opening of a medical device. Thedispenser 100 has a capillary tube 108 having a fluid outlet or orifice102, a fluid inlet 104, and an electrical cable 106. The piezoelectricdispenser 100 preferably includes a piezo crystal 110 within a housing112 for dispensing a fluid drop through the orifice 102. The crystal 110surrounds a portion of the capillary tube 108 and receives an electriccharge that causes the crystal shape to be perturbed. When the crystalcontracts inward, it forces a tiny amount of fluid out of the fluidoutlet 102 of the tube 108 to fill an opening 20 in a medical device. Inaddition, when the crystal expands outward, the crystal pulls additionalfluid into the tube 108 from a fluid reservoir connected to the inlet104 to replace the fluid that has been dispensed into the opening of themedical device.

In the exemplary embodiment as shown in FIG. 3, the micro-jettingdispenser 100 includes an annular piezoelectric (PZT) actuator 110bonded to a glass capillary tube 108. The glass capillary tube 108 isconnected at one end to a fluid supply (not shown) and at the other endhas an orifice 102 generally in the range of about 0.5 to about 150microns in diameter, and more preferably about 30 to about 60 microns.When a voltage is applied to the PZT actuator, the cross-section of thecapillary glass tube 108 is reduced/increased producing pressurevariations of the fluid enclosed in the glass capillary tube 108. Thesepressure variations propagate in the glass capillary tube 108 toward theorifice 102. The sudden change in cross-section (acoustic impedance) atthe orifice 102, causes a drop to be formed. This mode of producingdrops is generally called drop on demand (DOD).

In operation, the micro-jetting dispenser 100, depending on theviscosity and contact angle of the fluid, can require either positive ornegative pressure at the fluid inlet 104. Typically, there are two waysto provide pressure at the fluid inlet 104. First, the pressure at thefluid inlet 104 can be provided by either a positive or a negative headby positioning of the fluid supply reservoir. For example, if the fluidreservoir is mounted only a few millimeters above the dispenser 100, aconstant positive pressure will be provided. However, if the fluidreservoir is mounted a few millimeters below the dispenser 100, theorifice 102 will realize a negative pressure.

Alternatively, the pressure of the fluid at the inlet 104 may beregulated using existing compressed air or vacuum sources. For example,by inserting a pressure vacuum regulator between the fluid source andthe dispenser 100, the pressure may be adjusted to provide a constantpressure flow to the dispenser 100.

In addition, a wide range of fluids including or containing beneficialagents may be dispensed through the dispenser 100. The fluids deliveredby the dispenser 100 preferably have a viscosity of no greater thanabout 40 centipoise. The drop volume of the dispenser 100 is a functionof the fluid, orifice 102 diameter, and actuator driving parameter(voltage and timing) and usually ranges from about 50 picoliters toabout 200 picoliters per drop. If a continuous drop generation isdesired, the fluid may be pressurized and a sinusoidal signal applied tothe actuator to provide a continuous jetting of fluids. Depending on thebeneficial agent dispensed, each drop may appear more like a filament.

It may be appreciated that other fluid dispensing devices may be usedwithout departing from the present invention. In one exemplaryembodiment, the dispenser is a piezoelectric micro-jetting devicemanufactured by MicroFab Technologies, Inc., of Plano, Tex. Otherexamples of dispensers will be discussed below with respect to FIGS.7-9.

The electric cable 106 is preferably connected to associated driveelectronics (not shown) for providing a pulsed electric signal. Theelectric cable 106 provides the electric signal to control thedispensing of the fluid through the dispenser 100 by causing the crystalshape to be perturbed.

FIG. 4 shows an expandable medical device in the form of a stent 140receiving a drop 120 of a beneficial agent from a piezoelectricmicro-jetting dispenser 100. Into a whole 142. The stent 140 ispreferably mounted to a mandrel 160. The stent 140 may be designed withlarge, non-deforming struts and links (as shown in FIG. 1), whichcomprise a plurality of openings 142 without compromising the mechanicalproperties of the struts or links, or the device as a whole. Theopenings 142 serve as large, protected reservoirs for delivering variousbeneficial agents to the device implantation site. The openings 142 maybe circular, rectangular, or D-shaped in nature and form cylindrical,rectangular or D-shaped holes extending through the width of the stent140. In addition, openings 142 having a depth less than the thickness ofthe stent 140 may also be used. It may be appreciated that other shapedholes 142 may be used without departing from the present invention.

The volume of the hole 142 will vary depending on the shape, depth andsize of the hole 142. For example, a rectangular shaped opening 142having a width of 0.1520 mm (0.006 inches) and a height of 0.1270 mm(0.005 inches) will have a volume of about 2.22 nanoliters. Meanwhile, around opening having a radius of 0.0699 mm (0.00275 inches) will have avolume of about 1.87 nanoliters. A D-shaped opening having a width of0.1520 mm (0.006 inches) along the straight portion of the D, has avolume of about 2.68 nanoliters. The openings according to one exampleare about 0.1346 mm (0.0053 inches) in depth having a slight conicalshape due to laser cutting.

Although a tissue supporting device configuration has been illustratedin FIG. 1, which includes ductile hinges, it should be understood thatthe beneficial agent may be contained in openings in stents having avariety of designs including many of the known stents.

The mandrel 160 may include a wire member 162 encapsulated by an outerjacket 164 of a resilient or a rubber-like material. The wire member 162may be formed from a metallic thread or wire having a circularcross-section. The metallic thread or wire is preferably selected from agroup of metallic threads or wire, including Nitinol, stainless steel,tungsten, nickel, or other metals having similar characteristics andproperties.

In one example, the wire member 162 has an outer diameter of betweenabout 0.889 mm (0.035 inches) and about 0.991 mm (0.039 inches) for usewith a cylindrical or implantable tubular device having an outerdiameter of about 3 mm (0.118 inches) and an overall length of about 17mm (0.669 inches). It can be appreciated that the outer diameter of thewire member 162 will vary depending on the size and shape of theexpandable medical device 140.

Examples of rubber-like materials for the outer jacket 164 includesilicone, polymeric materials, such as polyethylene, polypropylene,polyvinyl chloride (PVC), ethyl vinyl acetate (EVA), polyurethane,polyamides, polyethylene terephthalate (PET), and their mixtures andcopolymers. However, it can be appreciated that other materials for theouter jacket 164 may be implemented, including those rubber-likematerials known to those skilled in the art.

In one exemplary embodiment, the wire member 162 is encapsulated in atubular outer jacket 164 having an inner diameter of about 0.635 mm(0.25 inches). The outer jacket 164 may be mounted over the wire member162 by inflating the tubular member to increase to a size greater thanthe outer diameter of the wire member 162. The tubular member can beinflated using an air pressure device known to those skilled in the art.The wire member 162 is placed inside of the outer jacket 164 by floatingthe outer jacket 164 of silicon over the wire member 162. However, itmay be appreciated that the wire member 162 may be encapsulated in anouter jacket of silicon or other rubber-like material by any methodknown to one skilled in the art.

In one exemplary embodiment for loading stents having a diameter ofabout 3 mm (0.118 inches) and a length of about 17 mm (0.669 inches), awire member 162 having an outer diameter of 0.939 mm (0.037 inches) isselected. In one example, the wire member 162 is about 304.8 mm (12inches) in length. The outer jacket 164 has an inner diameter of about0.635 mm (0.025 inches).

The expandable medical device or stent 140 is then loaded onto themandrel 160 in any method known to one skilled in the art. In oneexemplary embodiment, the stents 140 and the mandrel 160 are dipped intoa volume of lubricant to lubricate the stents 140 and the mandrel 160.The stents 140 are then loaded onto the mandrel 160. The drying of thestents 140 and the mandrel 160 create a substantially firm fit of thestents 140 onto the mandrel 160. Alternatively, or in addition todrying, the stents 140 may be crimped onto the mandrel 160 by a methodknown to one skilled in the art. The crimping ensures that the stents140 will not move or rotate during mapping or filling of the openings.

FIG. 5 shows a system 200 for loading a beneficial agent in anexpandable medical device. The system 200 includes a dispenser 210 fordispensing a beneficial agent into an opening of an expandable medicaldevice 232, a reservoir of beneficial agent 218, at least oneobservation system 220, and a mandrel 230 having a plurality ofexpandable medical devices 232 attached to the mandrel 230. The system200 also includes a plurality of bearings 240 for supporting therotating mandrel 230, a means 250 for rotating and translating themandrel 230 along a cylindrical axis of the expandable medical device232, a monitor 260, and a central processing unit (CPU) 270.

The dispenser 210 is preferably a piezoelectric dispenser for dispensinga beneficial agent into the opening in the medical device 232. Thedispenser 210 has a fluid outlet or orifice 212, a fluid inlet 214 andan electrical cable 216. The piezoelectric dispenser 200 dispenses afluid drop through the orifice 212.

At least one observation system 220 is used to observe the formation ofthe drops and the positioning of the dispenser 210 relative to theplurality of openings in the medical device 232. The observation system220 may include a charge coupled device (CCD) camera. In one exemplaryembodiment, at least two CCD cameras are used for the filling process.The first camera can be located above the micro-jetting dispenser 210and observes the filling of the medical device 232. The first camera isalso used for mapping of the mandrel 230 as will be described below. Asecond camera is preferably located on a side of the micro-jettingdispenser 210 and observes the micro-jetting dispenser 210 from a sideor orthogonal view. The second camera is preferably used to visualizethe micro-jetting dispenser during the positioning of the dispenserbefore loading of the medical device 232 with a beneficial agent.However, it can be appreciated that the observation system 220 caninclude any number of visualization systems including a camera, amicroscope, a laser, machine vision system, or other known device to oneskilled in the art. For example, refraction of a light beam can be usedto count drops from the dispenser. The total magnification to themonitor should be in the range of 50 to 100 times.

In one exemplary embodiment, a LED synchronized light 224 with the PZTpulse provides lighting for the system 260. The delay between the PZTpulse and the LED pulse is adjustable, allowing the capture of the dropformation at different stages of development. The observation system 220is also used in mapping of the mandrel 230 and medical devices 232 forloading of the openings. In one embodiment, rather than using a LEDsynchronized light 224, the lighting is performed using a diffusedfluorescent lighting system. It may be appreciated that other lightingsystems can be used without departing from the present invention.

A plurality of expandable medical devices 232 are mounted to the mandrel230 as described above. For example, a mandrel which is about 12 inchesin length can accommodate about 11 stents having a length of about 17 mmeach. Each mandrel 230 is labeled with a bar code 234 to ensure thateach mandrel is properly identified, mapped, and then filled to thedesired specifications.

The mandrel 230 is positioned on a plurality of bearings 240. As shownin FIG. 6, one example of the bearings 240 have a V-shaped notch 242.The mandrel 230 is positioned within the V-shaped notch 242 and securedusing a clip 244. The clip 244 is preferably a coil spring, however,other means of securing the mandrel within the V-shaped notch can beused including any type of clip or securing means can be used. Thebearings 240 may be constructed of a metallic material, preferablydifferent than the mandrel wire, such as stainless steel, copper, brass,or iron.

The mandrel 230 is connected to a means for rotating and translating themandrel 250 along the cylindrical axis of the medical device 232. Themeans for rotating and translating the mandrel 250 can be any type orcombination of motors or other systems known to one skilled in the art.

In one exemplary embodiment, the mandrel 250 and medical device 232 aremoved from a first position to a second position to fill the openings ofthe medical device 232 with the beneficial agent. In an alternativeexemplary embodiment, the system further includes a means for moving thedispensing system along the cylindrical axis of the medical device 232from a first position to a second position.

A monitor 260 is preferably used to observe the loading of the medicaldevice 232 with a beneficial agent. It can be appreciated that any typeof monitor or other means of observing the mapping and loading processmay be used.

A central processing unit 270 (or CPU) controls the loading of themedical device 232 with the beneficial agent. The CPU 270 providesprocessing of information on the medical device 232 for the dispensingof the beneficial agent. The CPU 270 is initially programmed with themanufacturing specifications as to the size, shape and arrangement ofthe openings in the medical device 232. A keyboard 272 is preferablyused to assist with the loading of the CPU 270 and for input ofinformation relating to the loading process.

The medical devices 232 are preferably affixed to the mandrel 230 andmapped prior to the loading process. The mapping process allows theobservation system and associated control system to determine a preciselocation of each of the openings which may vary slightly from device todevice and mandrel to mandrel due to inaccuracies of loading the deviceson the mandrels. This precise location of each of the openings is thensaved as the specific map for that specific mandrel. The mapping of themandrel 230 is performed by using the observation system to ascertainthe size, shape and arrangement of the openings of each medical device232 located on the mandrel 230. Once the mandrel 230 including theplurality of medical devices 232 have been mapped, the mapping resultsare compared to the manufacturing specifications to provide adjustmentsfor the dispenser to correctly dispense the beneficial agent into eachof the holes of the medical device 232.

In an alternative exemplary embodiment, the mapping of the mandrel 230is performed on an opening by opening comparison. In operation, theobservation system maps a first opening in the medical device andcompares the mapping result to the manufacturing specifications. If thefirst opening is positioned as specified by the manufacturingspecifications, no adjustment is needed. However, if the first openingis not positioned as specified by the manufacturing specifications, anadjustment is recorded and an adjustment is made during the dispensingprocess to correct for the position which is different than as specifiedin the manufacturing specifications. The mapping is repeated for eachopening of the medical device until each medical device 232 has beenmapped. In addition, in one embodiment, if an opening is mapped and theopening is positioned pursuant to the manufacturing specifications, themapping process can be designed to proceed to map at every other openingor to skip any number of openings without departing from the presentinvention.

After the mandrel has been mapped, the medical device 232 is filled withthe beneficial agent based on the manufacturers' specification andadjustments from the mapping results. The CPU provides the programmeddata for filling of each medical device 232. The programmed dataincludes the medical device design code, date created, lot number beingcreated, number of medical devices 232 on the mandrel, volume of eachopening in the medical device 232, different beneficial agents to beloaded or dispensed into the openings in the medical device 232, thenumber of layers, drying/baking time for each layer, and any other data.

In one exemplary embodiment, the medical device 232 will have at least10 beneficial agent layers which will be filled including at least onebarrier layer, at least one therapeutic layer having a beneficial agent,and at least one cap layer. The beneficial agent layers may includelayers which vary in concentration and strength of each solution of drugor therapeutic agent, amount of polymer, and amount of solvent.

In operation, the operator will input or scan the bar code 234 of themandrel into the CPU 270 before the filling process begins. The initialfilling generally includes a mixture of polymer and solvent to create abarrier layer. Each of the openings are typically filled to about 80percent capacity and then the mandrel with the medical device 232 stillattached is removed from the system and placed into an oven for baking.The baking process evaporates the liquid portion or solvent from theopenings leaving a solid layer. The mandrel is typically baked for about60 minutes plus or minus 5 minutes at about 55 degrees C. To assist inerror prevention, the CPU software receives the bar code of the mandreland will not begin filling the second layer until at least 60 minutessince the last filling. The second layer and subsequent layers are thenfilled in the same manner as the first layer until the opening has beenfilled to the desired capacity. The reservoir 218 may also be bar codedto identify the solution in the reservoir.

The observation system 220 also may be utilized to verify that thedispenser 210 is dispensing the beneficial agent into the openingsthrough either human observation on the monitor 270 or via data receivedfrom the observation system and conveyed to the CPU to confirm thedispensing of the beneficial agent in the openings of the medical device232. Alternatively, refraction of a light beam can be used to countdrops dispensed at a high speed.

The dispensers 100 run very consistently for hours at a time, but willdrift from day to day. Also, any small change in the waveform willchange the drop size. Therefore, the output of the dispenser 100 can becalibrated by firing a known quantity of drops into a cup and thenmeasuring the amount of drug in the cup. Alternatively, the dispenser100 may be fired into a cup of known volume and the number of dropsrequired to exactly fill it may be counted.

In filling the openings of the medical device 232, the micro-jettingdispenser 100 dispenses a plurality of drops into the opening. In onepreferred embodiment, the dispenser is capable of dispensing 3000 dropsper second through a micro-jetting dispenser of about 40 microns.However, the drops are preferably dispensed at between about 8 to 20shots per hole depending on the amount of fill required. Themicro-jetting dispenser fills each hole (or the holes desired) byproceeding along the horizontal axis of the medical device 232. The CPU270 turns the dispenser 100 on and off to fill the openingssubstantially without dispensing liquid between openings on the medicaldevice. Once the dispenser has reached an end of the medical device 232,the means for rotating the mandrel rotates the mandrel and a secondpassing of the medical device 232 along the horizontal axis isperformed. In one embodiment, the medical devices 232 are stents havinga diameter of about 3 mm and a length of about 17 mm and can be filledin about six passes. Once the medical device 232 is filled, thedispenser 210 moves to the next medical device 232 which is filled inthe same manner.

The CPU 270 insures that the mandrel is filled accurately by havingsafety factors built into the filling process. It has also been shownthat by filling the openings utilizing a micro-jetting dispenser, theamount of drugs or therapeutic agent used is substantially less thancoating the medical device 232 using previously known method includingspraying or dipping. In addition, the micro-jetting of a beneficialagent provides an improved work environment by exposing the worker to asubstantially smaller quantity of drugs than by other known methods.

The system 200 also includes an electrical power source 290 whichprovides electricity to the piezoelectric micro-jetting dispenser 210.

The medical devices 232 may be removed from the mandrel by expanding thedevices and sliding them off the mandrel. In one example, stents may beremoved from the mandrel by injecting a volume of air between the outerdiameter of the wire member 162 and the inner diameter of the outerjacket. The air pressure causes the medical device 232 to expand suchthat the inner diameter of the medical device 232 is greater than theouter diameter of the mandrel. In one embodiment, a die is place aroundthe mandrel to limit the expansion of the medical device 232 as the airpressure between the outer diameter of the wire member 162 and the innerdiameter of the outer jacket 164. The die can be constructed ofstainless steel or plastics such that the medical devices 232 are notdamaged during removal from the mandrel. In addition, in a preferredembodiment, the medical devices 232 are removed four at a time from themandrel. A 12-inch mandrel will accommodate about 11, 3 mm by 17 mmmedical devices having approximately 597 openings.

FIG. 7 illustrates one exemplary embodiment of a dispenser 300 whichprecisely delivers drops by acoustic drop ejection. The dispenser 300includes an acoustic energy transducer 310 in combination with areplaceable fluid reservoir 320. The dispenser 300 releases a nanoliteror picoliter drop from a surface of the liquid in the reservoir 320accurately into an opening in the medical device 140 positioned in thepath of the drop.

The dispenser 300 operates by focusing acoustic energy from thetransducer 310 through a lens onto the surface of the fluid in thereservoir 320. The fluid then creates a mound at the surface whicherupts and releases a drop of a controlled size and trajectory. Oneexample of a system for focusing the acoustic energy is described inU.S. Pat. No. 6,548,308 which is incorporated herein by reference. Themedical device 140 and mandrel 164 may be moved or the dispenser 300 maybe moved to precisely control the dispensing of the drops into theopenings in the medical device.

Some of the advantages of the use of an acoustic dispenser 300 includethe ability to deliver more viscous fluids and the ability to delivervolatile fluids containing solvents. For example, the fluids deliveredby the dispenser 300 can have a viscosity of greater than about 40centipoise. The delivery of more viscous materials allows the use ofhigher solids content in the delivered fluid and thus, fewer layers. Thedrop volume when using the dispenser 300 is a function of the fluid andtransducer driving parameters and can range from about 1 picoliter toabout 50 nanoliters per drop.

The dispenser 300 also has the advantage of simple and fast transferbetween dispensed liquids since the reservoir is self contained and theparts do not require cleaning. In addition, no loss of drug occurs whenswitching between drugs.

The acoustic dispenser 300 delivers the drop in a straight trajectorywithout any interference from the side walls of the reservoir 320. Thestraight trajectory of the fluid drops allows the dispenser 300 tooperate accurately spaced away from the medical device to allow improvedvisualization.

FIG. 8 illustrates an alternate exemplary embodiment of a reservoir 400for an acoustic dispenser which may deliver compositions containingvolatile solvents. The reservoir 400 includes a vapor chamber 410 abovethe fluid chamber 420. The vapor chamber 410 retains evaporated solventvapor and reduces the rapid evaporation rate of the volatile solvents byproviding a high concentration of solvent vapor at the surface of theliquid.

The dispenser 500 of FIG. 9 uses a solvent cloud formation system tosurround a dispenser 510, such as the piezoelectric dispenser of FIG. 3,with a cloud of the same solvent used in the dispensed fluid to reducesolvent evaporation and fowling of the dispenser tip. In the FIG. 9example, the solvent cloud is created by a ring 520 of porous material,such as porous metal, through which the solvent is delivered by a feedline 530 from an auxiliary solvent source. The solvent evaporating fromthe porous material ring 520 creates a cloud of solvent directly aroundthe dispenser tip. The creation of a solvent cloud around a dispensertip reduces the solvent vapor concentration differential near the tip ofthe dispenser. Lowering this differential will increase the time thatthe dispenser may be left idle without clogging due to solventevaporation. This improves the robustness of the process.

Alternatively, or in addition to the solvent cloud formation systemshown in FIG. 9, other gases may be delivered to form a cloud orcontrolled local environment around the tip of the dispenser whichassists in dispensing and reduces clogging of the dispenser.

The gas delivered around the dispenser tip, called a shield gas, createsa desirable local environment and shields the dispenser tip and thedispensed fluid from gases which can be detrimental to the dispensingprocess. Systems for delivering shield gases are known in the fields ofwelding and laser cutting and can include one or more outlets, jets, ornozzles positioned close to the dispensing tip for creating a desiredlocal environment at the processing location. The term shield gas asused herein refers to a gas delivered locally around a work area tochange the local environment.

In one example, a shield gas is used with a biologic agent, such ascells, genetic material, enzymes, ribosomes, or viruses. The shield gascan include a low oxygen gas creating a reducing atmosphere used toprevent oxidation.

In another example, the presence of high humidity in the environmentincreases the water content in the liquid solution dispensed by thedispenser tip. The high water content caused by high humidity can causesome drugs to crystallize and clog the dispenser tip. This clogging dueto humidity is particularly seen where a lipophilic agent, such as oneor more of the drugs paclitaxel, rapamycin, everolimus, and other limusdrugs, is dispensed. Thus, a dry shield gas may be used to preventclogging. In addition, the use of one or more solvents in the dispensedfluid that absorb water from a high humidity environment may stimulatethe crystallization of the drugs caused by high humidity. For example,the solvent DMSO absorbs water in a high humidity environment andincreases the precipitation and crystallization of some agents. Thehumidity within the local environment surrounding the dispensing tip maybe controlled to provide a desired humidity level depending on theparticular beneficial agent combination used, for example, the localhumidity can be maintained below 45 percent, below 30 percent, or below15 percent.

Examples of dry gases which may be used as the shield gas includenitrogen; inert gasses, such as argon or helium; dry air; or acombination thereof. The term dry gas as used herein means a gas havinga water content of less than 10 percent, and preferably a dry gasselected has less than 1 percent water content.

The shield gas may be provided in a pressurized liquid form which isexpanded and vaporized when delivered as the shield gas. Alternately, ashield gas may be stored in a gaseous form or created by removal ofwater from air or another gas. The shield gas orifice for delivery ofthe shield gas should be positioned close to the dispenser tip, forexample within about 1 inch, preferably within about ¼ inch from thedispensing tip. The dispensing tip may also be surrounded on two or moresides by shields or shrouds which contain the shield gas creating alocal environment between the shields and surrounding the dispensingtip.

The shield gas dispensing system may be controlled based on a sensedcondition of the environment. For example, the shield gas flow rate maybe automatically controlled based on a humidity of the room or a localhumidity near the dispensing tip. Alternately, the shield gas may beautomatically activated (turned on or off) by a local humidity sensorwhich senses a humidity near the dispensing tip or an in room humiditysensor. The shield gas dispensing system may also be controlled based onother sensed conditions of the environment, such as oxygen content.

The shield gas dispensing system may substantially reduce clogging ofthe dispensing tip, particularly of a piezoelectric dispensing tip bycontrolling the local environment around the dispensing tip. This shieldgas may eliminate the need for careful control of environmentalconditions of the entire room. The system may economically preventclogging of the dispenser due to different clogging mechanisms includingcrystallization of agents, rapid evaporation of solvents, drying, andothers.

In the example below, the following abbreviations have the followingmeanings. If an abbreviation is not defined, it has its generallyaccepted meaning.

TABLE I Solutions Drug Polymer Solvent A None 4% PLGA 50/50 DMSO DMSO DA0.64% paclitaxel 8% PLGA 50/50 DMSO IV = 0.60 DD 0.14% paclitaxel 8%PLGA 50/50 DMSO IV = 0.59 L None 8% PLGA 50/50 DMSO IV = 0.59 DMSO =Dimethyl Sulfoxide IV = Inherent Viscosity PLGA =poly(lactide-co-glycolide)

TABLE II Layer No., Layer No. Solution this Solution 1 A 1 2 A 2 3 A 3 4A 4 5 A 5 6 A 6 7 A 7 8 A 8 9 A 9 10 DA 1 11 DA 2 12 DD 1 13 L 1

A plurality of medical devices, preferably 11 medical devices permandrel are placed onto a series of mandrels. Each mandrel is bar codedwith a unique indicia which identifies at least the type of medicaldevice, the layers of beneficial agents to be loaded into the opening ofthe medical devices, and a specific identity for each mandrel. The barcode information and the mapping results are stored in the CPU forloading of the stent.

A first mixture of poly(latide-co-glycolide) (PLGA) (BirminghamPolymers, Inc.), and a suitable solvent, such as DMSO is prepared. Themixture is loaded by drops into holes in the stent. The stent is thenpreferably baked at a temperature of 55 degrees C. for about 60 minutesto evaporate the solvent to form a barrier layer. A second layer is laidover the first by the same method of filling polymer solution into theopening followed by solvent evaporation. The process is continued until9 individual layers have been loaded into the openings of the medicaldevice to form the barrier layer.

A second mixture of paclitaxel, PLGA, and a suitable solvent such asDMSO forming a therapeutic layer is then introduced into the openings ofthe medical device over the barrier layer. The solvent is evaporated toform a drug filled device protective layer and the filling andevaporation procedure repeated until the hole is filled until thedesired amount of paclitaxel has been added to the openings of themedical device.

A third mixture of PLGA and DMSO is then introduced into the openingsover the therapeutic agent to form a cap layer. The solvent isevaporated and the filling and evaporation procedure repeated until thecap layer has been added to the medical device, in this embodiment, asingle cap layer has been added.

In order to provide a plurality of layers of beneficial agents having adesired solution, the reservoir is replaced and the piezoelectricmicro-jetting dispenser is cleaned. The replacement of the reservoir andcleaning of the dispenser (if necessary) insures that the differentbeneficial layers have a desired solution including the correct amountof drugs, solvent, and polymer.

Following implantation of the filled medical device in vivo, the PLGApolymer degrades via hydrolysis and the paclitaxel is released.

As inkjet printing technology is increasingly applied in a broader arrayof applications, careful characterization of its method of use iscritical due to its inherent sensitivity. A common operational mode ininkjet technology known as drop-on-demand ejection is used as a way todeliver a controlled quantity of material to a precise location on atarget. This method of operation allows for the ejection of individualor the ejection of a sequence (burst) of drops based on a timed triggerevent. The present invention describes an examination of sequences ofdrops as they are ejected, indicating a number of phenomena that must beconsidered when designing a drop-on-demand inkjet system. Thesephenomena appear to be driven by differences between the first ejecteddrop in a burst and those that follow it and result in a break-down ofthe linear relationship expected between driving amplitude and dropmass. This first drop, as quantified by high-speed videography andsubsequent image analysis, detailed below, may be different inmorphology, trajectory, velocity and volume from subsequent drops withina burst. These findings were confirmed orthogonally by both volume andmass measurement techniques which allowed for quantization down tosingle drops.

In an increasingly broad spectrum of applications, the ability toaccurately and repeatedly deliver nanogram quantities of a givensubstance to a precise target location is critical to the development ofnew technologies. While inkjet technology is most commonly associatedwith printing applications, it has recently been utilized in a number ofother areas, including the manufacturing of medical devices for thedeposition of solutions containing polymers, drugs or combinations ofthe two.

Inkjet technology is based on acoustic principles and has been describedin great detail previously. The typical inkjet dispenser comprises ahollow glass tube with a piezoelectric element surrounding its outerdiameter. This piezoelectric element is dimensionally perturbed byincreasing and decreasing driving amplitudes, which expand and contractits diameter, respectively. These expansions and contractions producepressure waves within the glass tube which, in the correct combinationand timing, result in drop ejection. A typical driving waveform isillustrated in FIG. 10 with the relevant parameters labeled. Typicalparameters for the solution utilized herein were a 3 micro second risetime, a 20 micro second dwell time, a 3 micro second fall time and a 26volt amplitude driver at a frequency of 2.8 kHz. While all of theseparameters will have some impact on drop mass, driving amplitude is thedominant factor and is thus the primary control mechanism of ejectedmass.

Electrical parameters are just one subset of the factors that willdetermine ejected drop size and morphology; others include orifice sizeand condition, fluid properties, fluidic head and environmental factors.These factors, which are not of primary concern for this work, werecontrolled as closely as possible to avoid confounding effects.

Inkjet technology may be implemented in two main operational modes;namely, continuous and drop-on-demand. In continuous operation, driveelectronics provide a constant set of driving waveforms, resulting indrops that are dispensed continuously at a fixed frequency. Because itmay be undesirable for all of these drops to reach the target, the dropsare often charged via an electrostatic field and then deflected usinganother field to control their trajectory. In this way, the number ofdrops that reach the target may be controlled through fluctuations inthe electric field.

Since the inclusion of these systems significantly increases theircomplexity and cost, many applications choose instead to operate inkjetsin the drop-on-demand mode. In this mode, drive electronics only delivera set number of drive waveforms upon triggering, resulting in acontrollable number of drops reaching the target. This sequence of dropsmay then be triggered to dispense only when the desired target locationis in position, eliminating the need to deflect unwanted drops. Manyapplications use this method to deliver small quantities of drops tovarious points along a target surface, such as solder points inelectronic circuits and reservoirs in drug-eluting stents as describedherein. The number of drops dispensed at each trigger event may bemodified to control the final amount of substance that is dispensed andmay even be adjusted in real-time in a closed-loop controlled system toaccount for process drift or sudden changes in ejected mass.

To control the total amount of material delivered to the target,drop-on-demand operation allows for adjustment of drop size as well asthe number of drops delivered per trigger. However, there has been verylimited work conducted to characterize how changes to the number ofdrops delivered per burst might affect ejection behavior. Using dropnumber as a control for the quantity of material delivered to locationsalong a target assumes that each drop is equal in mass, implying alinear relationship between quantity of drops and total ejected mass.The present invention challenges this assumption and shows how,specifically, the first ejected drop may be different in both qualityand quantity from those that come after it, resulting in a non-linearresponse between number of drops in a burst and ejected mass. Thisdifference is also affected by the driving waveform, adding anotherlayer of complexity to drop-on-demand inkjets that must be taken intoaccount when designing such a system.

A commercially-available drop-on-demand inkjet system from MicroFabTechnologies was employed in the study described herein. The inkjet headwas a low-temperature unit with a 40 μm orifice diameter (MicroFabMJ-AB-63-40, MicroFab Technologies, Plano, Tex.) and was driven using aJetDrive III electronics control unit, which was connected to a standardcomputer running the JetServer software. Since triggering through theJetServer software is limited by bus rates and software-related cycletimes to approximately 250 ms, two JetDrive units were connected in acascading configuration. In this way, the first control unit was used toset the driving waveform parameters for drop ejection as well as thenumber of drops required per burst, while the second unit controlled thenumber of bursts to be dispensed, as well as the interval between them.This is described pictorially in FIG. 11 along with an example of aresulting set of waveforms. As illustrated in FIG. 11, a secondarycontrol unit is used to trigger a primary control unit, which isconnected directly to the inkjet head.

The liquid used in these experiments was a solution of drug and polymerdissolved in dimethyl sulfoxide (DMSO) as used in filling of NEVO™Sirolimus-eluting coronary stents. The addition of polymer resulted in anon-Newtonian fluid behavior. In order to reduce the viscosity of thesolution enough to allow for consistent dispensing, the solution washeated to 40 degrees C. as it passed through the inkjet unit, reducingthe viscosity to 4.95 cP and the surface tension to 41.5 dyn/cm. Thesolution vial was kept vented to atmospheric pressure and the solutionlevel was maintained at the same height as the tip of the inkjet toensure a consistent static fluid head.

A common underlying problem with inkjet dispensers of this type issolvent evaporation at the dispenser orifice potentially causingblockages at the jet tip as the solids in the solution precipitate outof solution. For the solution used in these studies, evaporation effectswere minimal due to the relatively high boiling point of DMSO (189degrees C.). However, DMSO is also highly hygroscopic and so waterabsorption was of greater concern than solvent evaporation due to theambient humidity conditions present during experimentation(approximately 30 percent). The net effect at the jet tip for eithersolvent evaporation or water absorption remains the same, though, asboth of these could drive the dissolved polymer and drug to precipitate,resulting in potential orifice blockages. To avert water absorption,nitrogen gas (99.998 percent high purity grade, Airgas, Inc., Radnor,Pa.) was kept continuously flowing around the orifice of the inkjet at1.0 L/min to exclude moisture from this area.

A combination of methods was used for the following experimentation,each providing a different means of quantifying differences betweendrops in a sequence. Initial work was performed by image analysis usingpictures captured by a high-speed video camera (Phantom v9.1, VisionResearch, Inc., Wayne, N.J.) with drops illuminated from behind using astandard projection-bulb lighting source. Images of drop sequences wererecorded at a frame rate matching the ejection frequency (2.8 kHz) tocapture one frame per drop. These images were then analyzed using theImageJ image-analysis software suite (National Institutes of Health,Bethesda, Md.). Drop volume was determined by first performing athreshold function on the image and then measuring the diameter of thedrop, from which the volume could be calculated. The camera and lenssystem was calibrated using an N.I.S.T. traceable optical standard fromEdmund Optics (Barrington, N.J.). Images were recorded at a sufficientdistance from the jet tip to allow vibrations in the drop caused byPlateau-Rayleigh instability to be damped (approximately 1 mm) tomaximize sphericitiy.

More sensitive drop mass quantification was carried out by means of UVspectroscopy. In this method, a number of drops (between 1 and 5) weredispensed into 100 μL of MilliQ de-gassed de-ionized water and thentransferred by pipette to an Agilent quartz Ultra-micro 10 mm pathlength cuvette. These samples were analyzed for absorption at 208 nm, anabsorbance peak associated with DMSO, from which the concentration ofDMSO, and subsequently drop volume, could be calculated through apre-determined standard curve. While this method's repeatability (4.1percent RSD for 1 to 5 drops) could not match that of weighing largernumbers of drops (0.26 percent RSD at 200 drops), it provided superiorsensitivity for small drop counts.

The final method employed was to dispense a larger number of drops intoa small weighing vessel (VWR Aluminum Micro Weighing Dishes). Thisvessel was then weighed on a Mettler-Toledo UMX2 sub-microgram balanceimmediately after capture to limit solvent evaporation and moistureabsorption. Both of these potentially mitigating factors wereexperimentally measured and determined to be sufficiently low to allowfor accurate measurements. Due to the limits of this balance, thismethod required a larger quantity of drops to be dispensed (greater than1000 drops) to provide adequate precision (repeatability of 0.26 percentRSD at 2000 drops).

These methods were used in combination to analyze various aspects of theeffect of first drop dissimilarities on jet performance. High-speedvideography provided qualitative assessments of drop morphology as wellas trajectory and velocity measurements, UV spectroscopy providedprecise quantization of small volumes of liquid (down to single drops onthe order of 90 pL) and mass determination by microbalance providedrapid analysis of large sample sizes while maintaining adequateprecision.

The combination of methods described above allowed for careful analysisof inkjet performance in drop-on-demand scenarios. Inkjets operated inthis mode deliver bursts of drops separated by a controllable timeinterval, which is useful for delivering more than one drop to differentpoints along a target. In this study, individual drops within thesebursts were analyzed for morphology, trajectory, velocity and volume todetermine how differences between them might affect ejection behavior.Sequences of drops were analyzed in this way for morphology, trajectory,velocity and volume with specific attention to how these were differentwithin a sequence.

Images captured by high-speed videography were the first indication thatejection of this solution did not yield identical drops when dispensedin a burst. An example of a burst of 5 drops is illustrated in FIG. 12,which demonstrates the dissimilarity in morphology and velocity. Theseimages were captured with a shutter speed of 2 micro seconds at a rateof 2800 fps and illustrate the dissimilarity between drops in asequence. The first drop is travelling faster as evidenced by itsdistance from one jet tip relative to the later drops and has a trail ofsmall satellite drops tailing it which is inconsistent with themorphology of later drops. While these attributes are very consistentfor drops 2 through 5, the first drop does not match this behavior,exhibiting higher ejection velocity and a tail of smaller satellitedrops. Long tails of satellite drops are undesirable from a targetingperspective and may also contribute to variability in ejected mass.

Image analysis provided quantization of drop volume for a large set ofimages such as those shown above. High-speed video was collected for 25sequences as described and illustrated herein and analyzed for mean dropmass, with the results illustrated in FIG. 13. Specifically, imageanalysis for high speed videography of 25 sets of bursts of 5 drops,with adjacent bursts separated by 30 ms was performed. Edge detectionwas performed to determine drop diameter from which drop mass wascalculated. Mean and two standard derivations are shown. For thisparticular set of driving conditions (dwell time=23 μs, amplitude=20 V),drop mass was found to increase with order of ejection, with the fifthdrop 10.3±2.2 percent larger than the first. Due to this effect, burstsof drops ejected in this manner would exhibit increasing average dropmass as a function of quantity of drops in a burst, resulting in therequirement for additional jet calibration activities to accuratelypredict ejected mass.

In order to eliminate the possibility that solution effects were drivingthis phenomenon, limiting its applicability to the liquid used in thesestudies, high-speed videography was repeated with a pure solvent, inthis case de-ionized water (driving parameters of 18 μs dwell time, 12 Vamplitude). While these images are not illustrated, similar behavior wasobserved with the first in a burst of drops exhibiting increasedvelocity as well as morphology inconsistent with those that followed it.Therefore, this behavior must not be a result of the non-Newtonianbehavior of the polymer solution and a more fundamental effect presentduring ejection of various fluids.

While gravimetric measurements by microbalance were only useful fornumbers of drops above 1000, it did provide an efficient way to measurea large numbers of samples. In these studies, the average drop weightwas determined by accumulating sufficient drops for precise measurementusing large sets of bursts and varying the number of drops per burst.Shown in FIG. 14 is one example of this, plotting average drop mass as afunction of driving amplitude for 5 and 800 drops per burst. While thelarger drop number demonstrates excellent linearity the smaller numbershows a non-linear behavior, with low and high amplitudes having thesame slope but different from each other, Regions A and C respectively,with a middle transition region, Region B. While this behavior hasroutinely been reported to be linear, this plot clearly indicates thatthis is not always the case. For the liquid used in these studies,smaller drop sequences display a non-linear behavior with three distinctregimes within this amplitude range: low (Region A) and high (Region C)amplitudes have the same slope but different intercepts while atransition zone (Region B) between these has a different slope andintercept.

This non-linearity is critical for applications in which the calibrationof an inkjet device needs to be highly accurate (e.g. delivery of anactive pharmaceutical ingredient). Under some circumstances, it may beattractive from a process design standpoint to calibrate an inkjetdevice by dispensing a large number of drops in one sequence instead ofin smaller, more process-reflective bursts, for instance for improvedcalibration precision or to reduce calibration time. However, this dataindicates that this is not always an appropriate solution, as averagedrop mass will change with the number of drops dispensed per burst.Thus, a truly accurate calibration may only be achieved by dispensingthe same number of drops per burst as used in the actual process.

Because these curves intersect at one particular driving amplitude, onemight also consider operating the inkjet at this setting and not takingthis effect into account. However, it should be noted that for thisliquid, this transition region did not occur at the same amplitude overa period of days. That is, the amplitude at which these curvesintersected changed +/−2 volts over a period of a week. The cause ofthis is not clear; however, from a practical standpoint, thisrelationship would have to be re-established at appropriate timeintervals in order to ensure that this cross-over amplitude has notchanged.

In order to understand the effects driving this behavior, it wasnecessary to repeat the high-speed videography described above fordriving amplitudes above the transition zone. This was not feasible,however, since drop morphologies were highly irregular at theseamplitudes with non-spherical drop morphologies and many satellitedrops. Because of this, image analysis was not able to producesufficiently accurate results. Instead, results were obtainedgravimetrically by determining average drop mass as a function of dropsper burst. In order to achieve this, the same total number of drops wasdispensed, in this case 1800, but with different numbers of drops ineach burst.

The results are illustrated in FIG. 15 which is a plot of average dropmass as a function of quantity of drops in a burst. The resultingaverage drop mass behavior illustrates the dissimilarity of the firstdrop from those that follow. Inkjet parameters were given as 18 microseconds dwell time, 38 v amplitude and 2.8 kHz driving frequency with a30 ms delay between sequences. Error bars indicate two standarddeviations. These results, illustrated in FIG. 15 appear to contradictthe high-speed videographic data presented in FIG. 13. However, thisstudy was performed with different inkjet parameters, such that it wasoperating above the transition zone (Region C) identified in FIG. 14. Asa result, the first drop is now shown to have significantly higher massthan subsequent drops whereas at lower amplitudes (Region A) it hadlower mass (see FIG. 13). As a result of this first drop dissimilarity,small and large burst sizes produced at the same driving amplitude wouldbe expected to demonstrate different average drop masses since smallbursts would be heavily influenced by the first drop while large enoughsets would mask this effect.

This finding, then, corroborates the data presented in FIG. 14. Withlarge sets of drops, drop weight changes linearly with driving amplitudesince this set is large enough to overcome the effect of the first drop.However, smaller sets show much more sensitivity to this effect.Further, the mass of the first drop is also very sensitive to drivingamplitude, much more so than later drops in a sequence. As a result, thefirst drop is smaller than subsequent drops for low amplitudes andlarger for high amplitudes. This, then, produces the non-linear behaviordescribed above.

While the microbalance measurements above seemed to identify thephenomenon driving this effect, there was no direct measurement ofindividual drop mass and so confounding effects, such as different heattransfer profiles to the jet over the sample collection time(potentially caused by different bulk mass flow rates due to the varyingnumber of drops in a burst) could have been introduced. A confirmatorystudy was therefore performed using UV spectroscopy, which was sensitiveenough to quantify single drops. This method allowed for accuratequantization of single bursts of drops instead of larger multiple ofthem, as required for gravimetric measurements.

While the repeatability of the UV method to measure the mass of smallquantities of drops (1 to 10) was not as robust as the gravimetricmethod (which required close to 2,000 drops), the sensitivity of the UVmethod allowed for mass determinations of single drops, allowing forverification of trends seen in other methods without confoundingfactors. In this case, as illustrated in FIG. 16 with the jet operatingin Region C of FIG. 14, these data support earlier results, indicatingthat the average mass of the first drop in a sequence is larger thanlater drops with the average drop mass leveling out around the thirddrop. Absorbance spectra were analyzed at 208 nano meters aftersubtracting water blank to detect DMSO. Mass was calculated fromconcentration which was determined through a standard curve fromabsorbance data. Mean values of 10 samples and two standard deviationsare shown in FIG. 16.

Previous studies have indicated the existence of what is often termedthe “first drop problem,” but this appears to be a phenomenon of adifferent time scale than what is presented herein. Thecommonly-referenced first drop problem refers to clogging or misfiringof an inkjet due to solvent evaporation at the orifice. Depending on thesolvent utilized, this effect would take on the order of seconds orminutes to present itself at a level significant enough to have thiskind of impact. However, the current effect is seen in every sequencesof drops with only a 30 ms interval between them, indicating aphenomenon beyond solvent evaporation or, more relevant to the currentscenario, water absorption. This effect, then, is hypothesized to becaused by a combination of effects including a) acoustic instability inthe channel of the inkjet, a result of insufficient time for regularacoustic reverberations to establish themselves within this channel, andb) orifice wetting effects, a result of fluid build-up around the jetorifice, which would occur only after drops begin to be dispensed. Inthe case of the unstable acoustics, after the 30 ms interval betweenbursts, these acoustic reverberations would be sufficiently damped forthis first drop phenomenon to reestablish itself, resulting in itsobservation in every burst of drops. A similar explanation would followfor the surface wetting case: a 30 ms delay would be sufficient to allowliquid that had collected around the orifice during ejection to be drawnback into the inkjet channel, leading to its repetition at the beginningof each burst. Attempts to limit solvent evaporation, as has beensuggested elsewhere, would not ameliorate either of these problems, asevidenced by this effect's presence even during ejection of pure water.

Drop-on-demand operation of inkjet devices provides a simple way toprecisely control the quantity of material reaching a target. However,it has been shown here that significantly more characterization isrequired to implement drop-on-demand dispensing than continuousdispensing operations. This is largely a result of the dissimilaritybetween the first drop ejected and subsequent drops, where the firstdrop is often different in morphology and trajectory, both of whichwould affect the ability to accurately reach the target, as well as inmass, which would impact dispensing accuracy. This will be of greatestconcern to applications in which small quantities of drops are thedeposited on various points along a target, as it is small drop burststhat are most sensitive to effects introduced by the first drop. Becausethe size of the first drop relative to those that follow is a functionof driving amplitude, neither the direction nor the magnitude of thebias introduced by this effect will be consistent and, thus, cannot beaccounted for mathematically. While deflecting this first drop toprevent it from reaching the target would be the ideal solution, inpractice this may be difficult to achieve due to rapid ejectionfrequencies and the added complexity this would introduce into thesystem. Instead, a carefully-designed dispensing protocol backed bythorough inkjet characterization for the particular solution of interestis the recommended method to account for these effects.

Since individual drops weigh in the range of 10 nano grams to 1 microgram, it is very difficult to determine their mass accurately, even inoff-line mode. This problem is further complicated by complex geometryand machine design used for actual deposition of drops. Hence on-linemeasurement of drop size and feedback control during deposition isextremely challenging. As a result, a calibration scheme is employedwhere a large number of drops (5000 to 20000) is collected and weighedto determine the average mass of ejected drops. This scheme assumes thatthe drop mass remains the same no matter how many drops are ejected.Because of the discrepancy between calibration and actual deposition asdescribed herein, the actual product does not receive the correct amountof the desired substance.

As described above, further complications with this discrepancy werediscovered. It has been found that the weight of the first few dropschanges as a function of the voltage amplitude used to create thesedrops. Hence the difference between the average mass calculated usingthe above calibration procedure and the average mass of first the 1 to20 (approximately) drops changes as a function of voltage amplitude.This is graphically depicted in FIG. 14.

This happens because, within a sequence of drops, the weight ofindividual drops gradually increases and then plateaus out whenoperating in region A (see FIG. 17 which graphically illustrates onedrop mass as a function or order of rejection within a burst for RegionA of FIG. 14). The first of any burst of drops is significantly moresensitive to driving amplitude than later drops in a burst with its massincreasing much more rapidly than later drops as a function of drivingamplitude. Thus, at amplitudes in Region C, the first drop in each burstis much larger than later drops. For small bursts of drops (e.g. the 5drops per burst shown in FIG. 14), this larger first drop has a largeimpact and increases the average mass of the burst. However, for largerbursts of drops (e.g. the 800 drops per burst shown in FIG. 14) thiseffect is masked by the averaging effect of dispensing so many drops andso the response is linear. Because of this averaging effect, the averagedrop mass will be a function of the number of drops in a burst with theeffect of the first drops being slowly diminished with larger and largerdrop counts. Hence, a constant offset cannot be used to compensate forthe discrepancy between calibration and actual drop depositionapplication since it will depend on how many drops are dispensed.

A number of methods may be utilized to correct for the first drop effectand achieve the correct amount of dispensed material during dropdeposition applications. The present invention is directed to methodsfor depositing the exact same amount of a particular substance atvarious well defined locations on an object of interest. In theexemplary embodiment described herein, the well defined locations arethe reservoirs and the object of interest is a stent. As describedabove, the jet deposits a number of drops at the location and theneither the jet moves or the object moves so that either way the jet isover the next location.

As illustrated in FIG. 17, which illustrates the average drop mass foran entire sequence of drops as a function of time between adjacentbursts, depending on the region of operation (FIG. 14), the drop massmight first increase or decrease and then plateau out. Therefore, inaccordance with a first exemplary method if the burst frequency and thejet/object movement can be controlled so that Ts<Td, wherein Ts is thetime between two sequences of drops or the time needed to move the jetfrom location to location and Td is the time between the ejection ofadjacent drops in a sequence of drops, however, since an initial burstfrequency is used by the jets to generate drops, then Td equals 1/(burstfrequency), then the first exemplary method outlined below may beutilized to obtain the same exact total drop mass at each location.

In the first step, a large number of drops is collected so that at thestart of the filling process, the device is operating in the plateauregion. This should only require the collection of a few hundred to afew thousand drops. From this the average drop weight may be calculatedand this way the weight of the initial drops will not significantlychange the average. In the second step, a calculation of how many dropswill be needed to render the desired drop mass at each location isperformed. In the third and final step, when the jet is turned on tostart the actual drop deposition operation, the first few drops arecollected in a waste collection container until the plateau region isreached and drops are deposited at every location while ensuring thatTs<Td. Since operation is now at the plateau region, consistent dropmass will be ensured.

As it may be difficult to meet the condition of Ts<Td due to variousfactors including high dispensing frequency and limitations in servospeed and the like, a different methodology may be required. FIG. 18graphically illustrates the average drop mass for an entire sequence ofdrops as a function of time between adjacent bursts. FIG. 18 illustratesthat if enough time Tr, wherein Tr is the time needed for the first dropeffect to reset, is allowed between consecutive sequences of drops, thenthe first drop effect can reset. Accordingly, if Ts>Tr, then everysequence of drops will have the same total weight. In this instance, thesecond exemplary methodology set forth below may be utilized and obtainor achieve the same total drop mass at every location.

In the first step, a large number of drops is collected by depositingsequences of drops in a collection container. This has to be done formany different cases where the number of drops in a sequence is changedfrom 1 to a large number to determine where the plateau for drop weightis achieved while making sure that for all drop sequences Ts>Tr holds.Then the average drop weight for different drop numbers is determined.In the second step, a calculation is performed of how many drops will beneeded to render the desired mass at each location. In the third andfinal step, drops are deposited at every location of the object by usingthe selected drop number above and making sure that Ts>Tr.

If neither of these conditions can be met, the difference betweencalibration by large numbers of drops and the dispensing process atsmall numbers of drops will remain. However, this may be accounted forin one of two ways. Determine the relationship shown in FIG. 14 for thespecific process to understand the difference between the calibrationand the dispensing process. The third exemplary method outlined belowmay be utilized to compensate for this difference mathematically byeither applying more or less material than calculated by the calibrationprocess depending on whether operation is in Region A or C. Alternately,ensure that calibration and dispensing process are identical for allparameters, including the number of drops in a sequence, so as not tointroduce any bias. The first drop effect will still exist but it willbe identical in both the calibration and dispensing process so thetarget material to be delivered will still be accurately achieved.

In accordance with another exemplary embodiment, the sub-thresholdvoltage priming of ink-jet devices in accordance with the presentinvention serves as a mechanism to ameliorate the first drop effectdescribed herein. This first drop effect, as described herein, is anundesired consequence of the operation of inkjets or inkjet devices inthe drop-on-demand dispensing mode. This mode enables the dispensing ofa finite number of drops per trigger event. When operating in this mode,it has been noted that the first in a burst of drops is often differentin morphology, volume, trajectory and/or velocity as compared to otherdrops in the burst. This difference may be detrimental to processes thatrequire accurate aiming of droplets and/or precise control over theamount of substance ejected per trigger event, such as that of loading atherapeutic agent into or onto an implantable medical device.

Inkjet devices as briefly described above operate based on acousticprinciples to generate droplets. FIG. 19 illustrates a typical albeitsimplified inkjet dispensing element 1900. In the current configuration,an annular piezoelectric (PZT) element 1902 surrounds a hollow glasscylinder 1904. A first end 1906 of the hollow glass cylinder or channel1904 is connected to a solution reservoir, not illustrated, and isreferred to as the closed end, while a second end 1908 of the hollowglass cylinder or channel 1904 is a nozzle or an open end. A protectivecasing 1910 surrounds the hollow glass cylinder or channel 1904.

The PZT element 1902 is controlled via an electrical waveform generator,not illustrated, the leads 1912 of which are connected to the inner andouter diameters of the PZT element 1902. With this type ofconfiguration, the PZT element 1902 is dimensionally perturbed whenintroduced to positive or negative voltages. A rise in voltage causesthe PZT element's inner and outer diameter to expand while a negativevoltage will cause the PZT element's inner and outer diameter tocontract. The correct timing of these positive and negative expansionsproduces constructive acoustic waves within the hollow glass cylinder orchannel 1904 with sufficient energy to eject a droplet at the open endor nozzle 1908 of the hollow glass cylinder or channel 1904. A typicalelectrical waveform used to eject drops is illustrated in FIG. 10. Thereis a rise time, a dwell time and a fall time.

In the continuous mode of operation of inkjet devices, the waveformsillustrated in FIG. 10 follow one after the other at a precisepredetermined frequency creating a very stable acoustic environmentinside the inkjet channel (i.e. each subsequent acoustic wave isproduced in the same acoustic environment as the previous acousticwave). This is not the case in drop-on-demand operation mode, since inthis mode of operation, a finite number of drops are ejected followed bya delay time followed by another finite number of drops. This delay timebetween finite drop bursts may not match the frequency of operationwithin the burst of drops and may differ as the channel moves betweentarget locations. It is presently hypothesized that this inconsistentacoustic environment is a major contributing factor to the first dropeffect described herein.

Another potential contributor to this effect is the build-up of excessfluid around the orifice of the inkjet channel during ejection. Thisfluid build-up is a well-established phenomenon and is a result ofsurface-wetting characteristics and drop ejection rate. While thisexcess fluid is usually drawn back into the channel through capillaryforces, when the ejection rate is too high and/or the surface-wettingcharacteristics too unfavorable, excess fluid can build-up around theorifice quicker than may be drawn back. This fluid deposition maysignificantly alter ejected drops through surface tension effects,potentially altering both drop velocity and volume.

Before a sequence of drops is dispensed, the orifice is free of thesefluid deposits. However, once drop ejection begins, this fluid begins todeposit, affecting later drops. This orifice condition inconsistencybetween the first and later drops may also contribute to the first dropeffect described herein.

As both of these hypothesized factors relate to the differentenvironments between the first ejected drop and later drops, a method inaccordance with the present invention is presented herein to establish astable environment before the first drop is ejected. This may beaccomplished by introducing acoustic waves inside the inkjet channel atthe desired frequency of operation as described above, but at amagnitude just below that necessary to actually produce droplets. Atthis magnitude, the acoustic waves will travel back and forth inside thechannel establishing a stable acoustic environment. These waves may alsobe controlled to push fluid past the orifice of the channel but withinsufficient force to break the surface tension and actually produce adroplet. With this condition, any fluid wetting at the jet orifice willalso be established before drop ejection begins. Once a number of cyclesof this sub-threshold priming are accomplished, the voltage supplied tothe PZT element will be increased real-time without changing thefrequency of waveform delivery, which could disrupt the acousticenvironment in the inkjet channel. A schematic of a series of primingand drop-producing waveforms is illustrated in FIG. 21, with pulses A, Band C below the threshold voltage necessary to produce droplets (primingpulses) and pulses D and E above the threshold necessary to eject drops.As illustrated, the waveforms in FIG. 21 are each the same asillustrated in FIG. 10 but with pulses A, B and C having a voltageamplitude below that necessary to produce droplets.

The introduction of these priming pulses establishes a stable,repeatable acoustic and orifice wetting environment similar to thatpresent during drop-on-demand ejection. This priming minimizes oreliminates the first drop effect, which is highly undesirable in anydrop-on-demand application.

The introduction of an electrical pulse to the piezo element of aninkjet causes the fluid within the channel to be perturbed, resulting inacoustic waves within the fluid that reverberate back and forth due toreflection at each end of the channel. Although these waves eventuallydampen out, if a second electrical pulse is introduced before theexisting reflected waves have been fully dampened, the effects of thereflected waves and the newly introduced wave are additive. Under thesecircumstances, the effect of a second electrical pulse will not be thesame as the effect of a first electrical pulse due to the additiveeffect of the acoustic waves within the channel. Also, the effect ofadditional electrical pulses will not be same as prior electrical pulsesuntil each subsequent electrical pulse results in the same maximumacoustic wave amplitude as the prior pulse. Since the size and velocityof drops ejected from an inkjet correspond to the amplitude of theacoustic wave that reaches the ejection orifice of the inkjet, drop sizeand velocity for subsequent drops in a burst can be expected to changeuntil the acoustic wave amplitudes have stabilized. The use of primingpulses of the appropriate design can create stabilized acoustic waveamplitudes that are just below the threshold required for drop ejection.A subsequent change to electrical impulses of larger amplitude will thenresult in acoustic waves with sufficient amplitude for drop ejection,while minimally perturbing the existing acoustic environment within thechannel. This will result in series of ejected drops whereby the firstdrop will be minimally different in velocity or mass from subsequentdrops.

It is important to note that the local delivery of drug/drugcombinations may be utilized to treat a wide variety of conditionsutilizing any number of medical devices, or to enhance the functionand/or life of the device. For example, intraocular lenses, placed torestore vision after cataract surgery is often compromised by theformation of a secondary cataract. The latter is often a result ofcellular overgrowth on the lens surface and can be potentially minimizedby combining a drug or drugs with the device. Other medical deviceswhich often fail due to tissue in-growth or accumulation ofproteinaceous material in, on and around the device, such as shunts forhydrocephalus, dialysis grafts, colostomy bag attachment devices, eardrainage tubes, leads for pace makers and implantable defibrillators canalso benefit from the device-drug combination approach. Devices whichserve to improve the structure and function of tissue or organ may alsoshow benefits when combined with the appropriate agent or agents. Forexample, improved osteointegration of orthopedic devices to enhancestabilization of the implanted device could potentially be achieved bycombining it with agents such as bone-morphogenic protein. Similarlyother surgical devices, sutures, staples, anastomosis devices, vertebraldisks, bone pins, suture anchors, hemostatic barriers, clamps, screws,plates, clips, vascular implants, tissue adhesives and sealants, tissuescaffolds, various types of dressings, bone substitutes, intraluminaldevices, and vascular supports could also provide enhanced patientbenefit using this drug-device combination approach. Perivascular wrapsmay be particularly advantageous, alone or in combination with othermedical devices. The perivascular wraps may supply additional drugs to atreatment site. Essentially, any type of medical device may be coated insome fashion with a drug or drug combination which enhances treatmentover use of the singular use of the device or pharmaceutical agent.

In addition to various medical devices, the coatings on these devicesmay be used to deliver any number of therapeutic and pharmaceuticagents. Some of the therapeutic agents for use with the presentinvention which may be transmitted primarily luminally, primarilymurally, or both and may be delivered alone or in combination include,but are not limited to, antiproliferatives, antithrombins,immunosuppressants including sirolimus, antilipid agents,anti-inflammatory agents, antineoplastics, antiplatelets, angiogenicagents, anti-angiogenic agents, vitamins, antimitotics,metalloproteinase inhibitors, NO donors, estradiols, anti-sclerosingagents, and vasoactive agents, endothelial growth factors, estrogen,beta blockers, AZ blockers, hormones, statins, insulin growth factors,antioxidants, membrane stabilizing agents, calcium antagonists,retenoid, bivalirudin, phenoxodiol, etoposide, ticlopidine,dipyridamole, and trapidil alone or in combinations with any therapeuticagent mentioned herein. Therapeutic agents also include peptides,lipoproteins, polypeptides, polynucleotides encoding polypeptides,lipids, protein-drugs, protein conjugate drugs, enzymes,oligonucleotides and their derivatives, ribozymes, other geneticmaterial, cells, antisense, oligonucleotides, monoclonal antibodies,platelets, prions, viruses, bacteria, and eukaryotic cells such asendothelial cells, stem cells, ACE inhibitors, monocyte/macrophages orvascular smooth muscle cells to name but a few examples. The therapeuticagent may also be a pro-drug, which metabolizes into the desired drugwhen administered to a host. In addition, therapeutic agents may bepre-formulated as microcapsules, microspheres, microbubbles, liposomes,niosomes, emulsions, dispersions or the like before they areincorporated into the therapeutic layer. Therapeutic agents may also beradioactive isotopes or agents activated by some other form of energysuch as light or ultrasonic energy, or by other circulating moleculesthat can be systemically administered. Therapeutic agents may performmultiple functions including modulating angiogenesis, restenosis, cellproliferation, thrombosis, platelet aggregation, clotting, andvasodilation.

Anti-inflammatories include but are not limited to non-steroidalanti-inflammatories (NSAID), such as aryl acetic acid derivatives, e.g.,Diclofenac; aryl propionic acid derivatives, e.g., Naproxen; andsalicylic acid derivatives, e.g., Diflunisal. Anti-inflammatories alsoinclude glucocoriticoids (steroids) such as dexamethasone, aspirin,prednisolone, and triamcinolone, pirfenidone, meclofenamic acid,tranilast, and nonsteroidal anti-inflammatories. Anti-inflammatories maybe used in combination with antiproliferatives to mitigate the reactionof the tissue to the antiproliferative.

The agents may also include anti-lymphocytes; anti-macrophagesubstances; immunomodulatory agents; cyclooxygenase inhibitors;anti-oxidants; cholesterol-lowering drugs; statins and angiotens inconverting enzyme (ACE); fibrinolytics; inhibitors of the intrinsiccoagulation cascade; antihyperlipoproteinemics; and anti-plateletagents; anti-metabolites, such as 2-chlorodeoxy adenosine (2-CdA orcladribine); immuno-suppressants including sirolimus, everolimus,tacrolimus, etoposide, and mitoxantrone; anti-leukocytes such as 2-CdA,IL-1 inhibitors, anti-CD116/CD18 monoclonal antibodies, monoclonalantibodies to VCAM or ICAM, zinc protoporphyrin; anti-macrophagesubstances such as drugs that elevate NO; cell sensitizers to insulinincluding glitazones; high density lipoproteins (HDL) and derivatives;and synthetic facsimile of HDL, such as lipator, lovestatin,pranastatin, atorvastatin, simvastatin, and statin derivatives;vasodilators, such as adenosine, and dipyridamole; nitric oxide donors;prostaglandins and their derivatives; anti-TNF compounds; hypertensiondrugs including Beta blockers, ACE inhibitors, and calcium channelblockers; vasoactive substances including vasoactive intestinalpolypeptides (VIP); insulin; cell sensitizers to insulin includingglitazones, P par agonists, and metformin; protein kinases; antisenseoligonucleotides including resten-NG; antiplatelet agents includingtirofiban, eptifibatide, and abciximab; cardio protectants including,VIP, pituitary adenylate cyclase-activating peptide (PACAP), apoA-Imilano, amlodipine, nicorandil, cilostaxone, and thienopyridine;cyclooxygenase inhibitors including COX-1 and COX-2 inhibitors; andpetidose inhibitors which increase glycolitic metabolism includingomnipatrilat. Other drugs which may be used to treat inflammationinclude lipid lowering agents, estrogen and progestin, endothelinreceptor agonists and interleukin-6 antagonists, and Adiponectin.

Agents may also be delivered using a gene therapy-based approach incombination with an expandable medical device. Gene therapy refers tothe delivery of exogenous genes to a cell or tissue, thereby causingtarget cells to express the exogenous gene product. Genes are typicallydelivered by either mechanical or vector-mediated methods.

Some of the agents described herein may be combined with additives whichpreserve their activity. For example additives including surfactants,antacids, antioxidants, and detergents may be used to minimizedenaturation and aggregation of a protein drug. Anionic, cationic, ornonionic surfactants may be used. Examples of nonionic excipientsinclude but are not limited to sugars including sorbitol, sucrose,trehalose; dextrans including dextran, carboxy methyl (CM) dextran,diethylamino ethyl (DEAE) dextran; sugar derivatives includingD-glucosaminic acid, and D-glucose diethyl mercaptal; syntheticpolyethers including polyethylene glycol (PEO) and polyvinyl pyrrolidone(PVP); carboxylic acids including D-lactic acid, glycolic acid, andpropionic acid; surfactants with affinity for hydrophobic interfacesincluding n-dodecyl-.beta.-D-maltoside, n-octyl-.beta.-D-glucoside,PEO-fatty acid esters (e.g. stearate (myrj 59) or oleate),PEO-sorbitan-fatty acid esters (e.g. Tween 80, PEO-20 sorbitanmonooleate), sorbitan-fatty acid esters (e.g. SPAN 60, sorbitanmonostearate), PEO-glyceryl-fatty acid esters; glyceryl fatty acidesters (e.g. glyceryl monostearate), PEO-hydrocarbon-ethers (e.g. PEO-10oleyl ether; triton X-100; and Lubrol. Examples of ionic detergentsinclude but are not limited to fatty acid salts including calciumstearate, magnesium stearate, and zinc stearate; phospholipids includinglecithin and phosphatidyl choline; (PC) CM-PEG; cholic acid; sodiumdodecyl sulfate (SDS); docusate (AOT); and taumocholic acid.

In accordance with another exemplary embodiment, a UV-visiblespectroscopic method for quantitation of inkjet drop mass may beutilized to accurately and repeatedly deposit nanogram quantities of agiven substance at a target location. The general principle behind thismethod involves dispensing a known number of droplets of a solution ofinterest from the inkjet dispenser (herein referred to as the inkjetsolution) into a cuvette loaded with a precise amount of a givensolvent. An absorbance spectrum of the sample in the cuvette (hereinreferred to as the spectroscopic solution) is then measured and astandard curve is used to calculate the concentrations of the variouscomponents in the cuvette. The resultant concentrations may then beutilized to calculate the mass of the droplets dispensed from the inkjetsince the original volume of solvent in the cuvette is known.

The drop-on-demand apparatus used in the experiment describedsubsequently was a low temperature biologics inkjet (MJ-AB-63-40,MicroFab Technologies, Plano, Tex.) with a 40 μm orifice diameter. Thisdevice was controlled via a MicroFab JetDrive III electronics controlunit connected to a standard computer, through which electricalparameters for the driving waveform were configured. The inkjet solutionused in these experiments to create droplets comprised dimethylsulfoxide (DMSO), poly(lactic-co-glycolic acid) (PLGA) and sirolimus, arapamycin. An Agilent 8453 UV-Visible spectrophotometer (AgilentTechnologies, Santa Clara, Calif.) was used to measure absorbance datawhich was subsequently analyzed using the Agilent ChemStation softwaresuite.

De-ionized water (MilliQ or equivalent) was used as the solvent loadedinto the cuvettes for all samples in this experiment due to its lowcutoff wavelength and its ease of transport and disposal in amanufacturing environment. Samples were prepared and collected in glassscintillation vials and then transferred using a pipettor to one of twotypes of Agilent quartz cuvettes, depending on the quantity of dropletsbeing quantified. This cuvette required either a sample volume of 2 mLor 0.1 mL and so all samples described herein were dissolved in one ofthese volumes of water, which was degassed before use. Absorbance valueswere collected between 190 and 1100 nm at 1 nm increments with a 1 secintegration time.

Preliminary screening experiments were conducted to determine thefeasibility of measuring the components in solution at variousconcentrations so absorption was measured over a wide range ofwavelengths. Later experiments used absorbance values only at awavelength of 208 nm, corresponding to the peak absorbance for DMSO.When more precise absorbance determinations were required, a cuvette wasfirst tared on an analytical balance (Mettler Toledo XP205 DeltaRange)then loaded with 1 mL of de-ionized water and weighed on the samebalance. This allowed for more accurate determination of concentrationsince the solvent volume was determined both volumetrically as well asgravimetrically.

This experiment was designed for the development of a UV-visiblespectroscopic method for characterization of inkjet microdispensers. Ofinterest were two primary goals: to improve upon the detection limits ofcurrent methods by being able to quantify inkjet output in the hundredsof drops instead of thousands and to aid in the assessment of inkjetsolution mixing properties after ejection. The first goal required thetracking of a single species in the spectroscopic solution and measuringabsorbance while carefully controlling for other factors. The secondgoal was an extension of the first and sought to track multiple speciesin the spectroscopic solution to ensure homogeneity and gain insightinto possible inkjet-induced non-uniformities such as de-mixing andprecipitation.

Initial feasibility experiments sought to determine whether thesubstances in the inkjet solution were detectable and whether theassociated concentrations would yield absorbance values adequate toquantify the species in question without detector saturation. Toaccomplish this, a wide range of spectroscopic solution concentrationswere measured in the wavelength range of 190 to 1100 nm using solutionsof only DMSO in water. As DMSO constituted 86 percent of the inkjetsolution by volume, it was postulated that this would exhibit thestrongest absorbance and would constitute the species of greatest impactin determining jet output.

FIG. 21 illustrates the first of such experiments, in which variousconcentrations of DMSO were dissolved in de-ionized water. Dropquantities above 1900 were found to saturate the detector readings andthose below 57 did not exhibit sufficient absorbance for quantitation.Concentrations between these values produced an absorbance peak at 208nm, within the desired absorbance range for the spectrometer, with thiswavelength coinciding with the theoretical absorbance of DMSO. Despitethe proximity of the absorbance peak associated with DMSO to the cutoffwavelength of water, the peak was distinct enough to justify furtherinvestigation.

The use of a solvent as the species of interest for quantitationintroduces problems associated with evaporation as this will hinder thismethod's ability to accurately assess the actual amount of substancebeing ejected from the inkjet. While many applications would need totake this into account, this is of little concern for this study due toDMSO's relatively high boiling point (189° C.) and resultant slowevaporation rate. Further preventing excess evaporation is the minimalamount of time the ejected drops spend in the air before reaching thecollection liquid (approximately 10 ms). Of greater concern when workingwith DMSO is moisture absorption from the laboratory environment due toits hygroscopic behavior. However, moisture absorption has no effect onthe DMSO peak absorbance wavelength and estimated moisture absorptionquantities were calculated to have no significant effect on the aqueousspectroscopic sample volume with no measurable impact on concentrationcalculations.

Having determined the ideal concentration range for the primarycomponent of the inkjet solution, drug and polymer were added to thissolution and a similar screening study was conducted to determineabsorbance peaks associated with these other species. A typical spectrumfor the full solution is shown in FIG. 22.

In addition to the absorbance peak at 208 nm associated with DMSO andPLGA, a triplet peak also exists centered at 281 nm with shoulder peaksat 271 and 293 nm. This triplet matches the theoretical absorbancewavelength and behavior of rapamycin but exhibits an absorption maximumapproximately one third of the absorbance of DMSO. However, since theabsorbance peaks of both species can be captured at the samespectroscopic sample concentration, simultaneous assessments ofconcentrations for these two species are feasible. While both DMSO andPLGA absorb at 208 nm, the concentration of PLGA in the spectroscopicsolution was below the detection limit and as such did not contribute tothe absorbance at this wavelength. A spectroscopic sample concentrationof 10 ug/mL (equivalent to 200 drops dispensed from the inkjet) waschosen for inkjet performance characterization based on the balancebetween maximizing absorbance strength and minimizing drop quantity.

This sample concentration was associated with different quantities ofdrops ejected from an inkjet device depending on the cuvette being used.For a larger cuvette on the order of a 2 mL sample volume, this amountedto approximately 200 drops for the current inkjet configuration.However, for a micro-cuvette on the order of a 100 μL sample volume,this concentration amounted to a single drop for the current inkjetconfiguration. While some loss of fidelity is expected formicro-cuvettes due to their short path length, the present methodexhibited a 193:1 signal-to-noise ratio for a single drop dispensed intoa micro-cuvette.

The present invention may be simply described as a method fordetermining at least one of the drop mass or concentration of dissolvedsolutes for one or more drops of a liquid of interest dispensed from aninkjet dispensing device.

Droplets dispensed from an inkjet typically consist of a predominantliquid component which may or may not additionally comprise variousdissolved minor solute components, for example, polymer or active drugor therapeutic agent substance. Careful preparation of this dispensingsolution means that the precise mass ratios of all components will beknown. For UV determination of the mass of one or more droplets, anappropriate UV absorption wavelength for the component of interest needsto be determined. This involves the preparation of a number ofsolutions, the first of which will be the predominant liquid component,the second of which will be the predominant liquid component incombination with the first minor solute component if present, the thirdof which will be the predominant liquid component in combination withthe second minor solute component if present, etc. Each solution is thenplaced in a quartz cuvette which is illuminated by the UV light sourceof a UV/Vis spectrometer. As the light is transmitted through thecuvette, some light may be absorbed by the dispensing solutioncomponents in a wavelength-dependent manner. By investigating the UVlight absorption for each dispensing solution component between 190 and400 nm, the wavelength corresponding to maximum absorption, if present,may be established.

A standard curve is then prepared by diluting the dispensing solutioninto water at various precisely controlled dilution levels and measuringthe absorbance of each solution at the wavelength corresponding to themaximum absorbance of the component of interest. A plot of absorption atthis wavelength versus component concentration is then created. To avoidpossible absorbance instabilities, each solution may be sonicated beforeUV absorption determination to remove micro bubbles.

For determination of the mass of dispensed drop(s), one or more inkjetdroplets are dispensed into a UV cuvette comprising a preciselypredetermined quantity of water (typically quantified with a precisionmicrobalance). The absorbance value at the maximum absorption wavelengthcorresponding to the predominant liquid component is compared to itsstandard curve, which then allows determination of its concentrationwithin the cuvette. Since the quantity of water in the cuvette is known,the resultant concentration within the cuvette may be converted to thetotal mass of predominant liquid component dispensed. Subsequently,since the mass ratio for all components of the dispensing solution areknown, the total mass of dispensing solution dispensed in one or moredrops may be calculated.

Similarly, to determine the concentration of various minor solutecomponents, the absorption value at the maximum absorption wavelengthfor each minor component is compared to its standard curve allowing fordetermination of the concentration of each component individually in thecuvette. Using similar logic as above, concentration ratios for thesecomponents as well as mass per drop for each component may becalculated.

Although shown and described is what is believed to be the mostpractical and preferred embodiments, it is apparent that departures fromspecific designs and methods described and shown will suggest themselvesto those skilled in the art and may be used without departing from thespirit and scope of the invention. The present invention is notrestricted to the particular constructions described and illustrated,but should be constructed to cohere with all modifications that may fallwithin the scope of the appended claims.

What is claimed is:
 1. A method for determining at least one of the dropmass or concentration of dissolved solutes for one or more drops of aliquid of interest dispensed from an inkjet dispensing device, themethod comprising: depositing one or more drops of a first solvent froman inkjet dispensing device into a cuvette containing a predeterminedamount of a second solvent and mix; determining the UV-absorptionspectra of the resulting solution of the first and second solvents inthe cuvette; and calculating the mass of the one or more drops dispensedby the inkjet dispensing device by utilizing the UV-absorption spectraat the specific wavelength that corresponds to the first solvent andcomparing it to a predetermined calibration curve.
 2. A method fordetermining at least one of the drop mass or concentration of dissolvedsolutes for one or more drops of a liquid of interest dispensed from aninkjet dispensing device, the method comprising: depositing one or moredrops of a liquid of interest comprising a first solvent and one or moresolutes from an inkjet dispensing device into a cuvette containing apredetermined amount of a second solvent and mix into a resultingsolution; determining the UV-absorption spectra of the resultingsolution in the cuvette; calculating the mass of the one or more dropsdispensed by the inkjet dispensing device by utilizing the UV-absorptionspectra at the specific wavelength that corresponds to the first solventand comparing it to a predetermined calibration curve; and calculatingthe concentration of the one or more solutes in each of the one or moredrops dispensed by the inkjet dispensing device by utilizing theUV-absorption spectra at the specific wavelengths that correspond to theone or more solutes in the solution and comparing it to predeterminedcalibration curves.
 3. The method for determining at least one of thedrop mass or concentration of dissolved solutes for one or more drops ofa liquid of interest dispensed from an inkjet dispensing deviceaccording to claim 1, wherein the one or more solutes comprises at leastone of a polymer, a therapeutic agent or a combination of polymer andtherapeutic agent.
 4. The method for determining at least one of thedrop mass or concentration of dissolved solutes for one or more drops ofa liquid of interest dispensed from an inkjet dispensing deviceaccording to claim 3, wherein the therapeutic agent comprises arapamycin.
 5. The method for determining at least one of the drop massor concentration of dissolved solutes for one or more drops of a liquidof interest dispensed from an inkjet dispensing device according toclaim 3, wherein the polymer comprises PLGA.